1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to large relative aperture, high range zoom lenses of the rear focus type whose zoom ratio is 8 to 12 and F-number is 1.6 to 2 with a long back focal distance for use in photographic cameras, video cameras, or broadcasting cameras.
2. Description of the Related Art
For the zoom lenses in a photographic camera or a video camera, there have been previous proposals for focusing by moving other than the front or first lens unit. The so-called rear focus type has thus found its use in a variety of zoom lenses.
As compared with the zoom lens of the type in which focusing is performed by moving the first lens unit, the zoom lenses of the rear focus type generally have the advantages that the first lens unit gets a smaller effective diameter so as to facilitate minimization of the bulk and size of the entire lens system, and also that close-up photography, particularly photomacrography, becomes easy to do, and further that, since focusing movement is imparted to a lens unit of relatively small size and light weight, a smaller incentive suffices for driving it, making it possible to adjust the focus rapidly.
In application of such a rear focus type to the so-called 4-unit zoom lens, an example is shown in Japanese Laid-Open Patent Application No. Sho 63-44614 where a zoom lens comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the image shift with zooming and a fourth lens unit of positive refractive power, totaling four lens units, whereby the third lens unit is used for focusing. However, this zoom lens has to secure a space in which the third lens unit moves. So, there is a tendency to increase the physical length.
In Japanese Laid-Open Patent Application No. Sho 58-136012, the zooming section is constructed with three or more lens units, one of which is moved for focusing purposes.
In Japanese Laid-Open Patent Applications Nos. Sho 63-247316 and Sho 62-24213, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the second lens unit is moved to vary the focal length and the fourth lens unit is moved to compensate for the image shift with zooming, whereby focusing is performed by moving the fourth lens unit.
In Japanese Laid-Open Patent Application No. Sho 58-160913, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the first and second lens units are moved to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. Of these lens units, one or two or more lens units are moved for focusing purposes.
In Japanese Laid-Open Patent Application No. Sho 63-278013, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of negative refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the second lens unit is moved to vary the focal length and the fourth lens unit is moved to compensate for the image shift with zooming and also to effect focusing.
Another form of the 4-unit zoom lens is proposed in Japanese Laid-Open Patent Application No. Sho 57-111507, comprising, from front to rear, a first lens unit of positive refractive power which is stationary during zooming and movable for focusing, a second lens unit of negative refractive power and a third lens unit of positive refractive power, wherein the second and third lens units move in opposite directions to each other during zooming, whereby the third lens unit has two positive lens units arranged to move in differential relation to each other. With such a 4-unit zoom lens of plus-minus-plus-plus refractive power arrangement, according to this document, location of an aperture stop is made within the third lens unit.
Since, in this configuration, however, the second and third lens units move always in opposite directions, there is need to make wider room for the separation between the second and third lens units at the wide-angle end. Also, since the stop lies within the third lens unit, as this implies that at the wide-angle end, the position of the entrance pupil is nearest to the image side, such a zoom lens is not suited to a use for minimizing the diameter of the front lens members and the size of the entire lens system.
Then, because focusing is performed by the first lens unit, the diameter of the front lens members must be increased in order to admit a light beam going to the corners of the image frame at the minimum object distance. If attempts are made to minimize the bulk and size by adapting this zooming configuration to the rear focus type, however, a problem arises in that the refractive power arrangement is not found optimum, or the variation of aberrations with focusing is not corrected sufficiently well, as the rear focus type is used.
Also, even in Japanese Laid-Open Patent Application No. Hei 3-200113, a zoom lens of similar form is shown comprising, from front to rear, a positive first lens unit which is stationary during zooming, a negative second lens unit which moves back and forth to vary the focal length from one end to another, a positive third lens unit which moves in associative relation to the movement of the second lens unit, and a positive fourth lens unit which moves in part or in whole to compensate for the shift of the image plane with zooming.
According to this document, what is meant by the associative movement of the positive third lens unit with the movement of the second lens unit is done in order to reduce that amount of compensation for the image shift which is required for the fourth lens unit, in other words, to let the third lens unit share part of the compensating function. Concretely speaking, it is taken as desired that, when zooming in a region of from a middle focal length position to the telephoto end, the third lens unit moves from the image side to the object side.
With this arrangement, however, because the second and third lens units move in opposite directions, there is need to make wider room for the separation between the second and third lens units at the wide-angle end. As this implies that at the wide-angle end, the entrance pupil takes its place nearest to the image plane, there is a difficult problem of minimizing the diameter of the front lens member and the size of the entire lens system.
Again, Japanese Laid-Open Patent Application No. Hei 3-158813, too, discloses a similar zoom lens as comprising, from front to rear, a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit, the second and third lens units being axially moved to vary the focal length, and the aperture stop being moved in unison with the third lens unit.
According to this document, during zooming from the wide-angle end to the telephoto end, the separation between the second and third lens units decreases. The third lens unit carrying the aperture stop also takes the rearmost possible position when in the wide-angle end. So, that neighborhood of the third lens unit which contains the stop approaches nearest to the image plane in the wide-angle end, or in a slightly zoomed position therefrom, where the front lens members necessarily have a largest diameter. As this means that the entrance pupil goes up deep, a disadvantage is produced at reducing the diameter of the front members. Moreover, distortion gets large in the wide-angle end. With this distortion removed for good optical performance, therefore, a problem arises in that it is very difficult to shorten the diameter of the front members and reduce the size of the entire lens system.
In another Japanese Laid-Open Patent Application No. Hei 3-215810, a zoom lens of the rear focus type is proposed comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a stop, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, wherein during zooming from the wide-angle end to the telephoto end, the second lens unit moves toward the image side, while simultaneously moving the stop, the third lens unit and the fourth lens unit independently of each other so that each depicts a locus convex toward the object side, and wherein focusing is performed by moving the fourth lens unit.
In general, the use of the rear focus type in zoom lenses gives the advantages that, as has been described before, the entire lens system is minimized in bulk and size, and rapid focusing can be carried out, and further close-up photography becomes easy to do.
For the counterpart, however, the variation of aberrations with focusing increases largely, giving rise to a problem in that it becomes very difficult to obtain high optical performance throughout the entire focusing range in such a manner that the bulk and size of the entire lens system are limited to a minimum.
At the present time, for the video camera for public use, the image sensor is in the form of a single chip. In this case, the color separation prism is not needed that is commonly used in the multiple chip type cameras for professional use. The zoom lens for the single chip type camera has a relatively short back focal distance.
In the case of the multiple chip type, the color separation prism and its associated parts are arranged behind the photographic lens (zoom lens). For this reason, the zoom lens for the multiple chip type camera necessitates a relatively long back focal distance compared with the zoom lens for the single chip type video camera for public use.